Fill in the following blanks with suitable words :
Souring of milk is a _____change.

Explanation

To answer the question "Souring of milk is a _____ change," we need to determine the type of change that occurs when milk sours.

### Step-by-Step Solution:

1. **Understanding the Process**: When milk sours, it undergoes a transformation due to the action of bacteria. These bacteria feed on the lactose (the sugar present in milk) and produce lactic acid as a by-product.

2. **Identifying the Type of Change**:
- A **physical change** involves changes in physical properties without altering the chemical composition of the substance (like melting, freezing, or dissolving).
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